From Jeffrey E. Patterson, Ph.D. candidate, University of Calgary,(jeffreyp@pegmatite.com)

Drs. Martin, White, and London present interesting comments regarding some aspects of hollowtourmaline growth in pegmatites. However, they do not address the other primary questionsposed by Mr. Rose, to wit, what can he read to further his knowledge base and can hisobservations be useful for further exploration. The literature is replete with papers dealing withcriteria for exploration, including the use of mineralogical trends. Basic models, withassumptions, can be found in Cameron, et al., (1949), Jahns and Burnham, (1969), andSinkankas, (1988). An easily accessible bibliographical source on tourmalines and theirproperties is provided by Darrell Henry, at Louisiana State University,(http://www.geol.lsu.edu/henry/Research/tourmaline/bibliography/TurBibliography.htm).

From my current research, I have observed that recent investigation of tourmaline stability andalteration to other minerals is a little studied phenomenon. The changing acidity/alkalinity of thelate hydrothermal fluids circulating in pegmatites offers a unique opportunity to examine someof the basic characteristics of pegmatite formation. The work of Thomas, et al., (2003), providesseveral new insights to this problem. The example presented by Mr. Rose is exceptional in itscharacteristics, specifically with the apparent pristine nature of the remaining schorl, theterminated quartz on the exterior, and the terminated sphene on the interior. Dr. London’sFigures 3, from the Little 3 Mine in Ramona, and 4 and 5, from the Elizabeth R Mine in Pala,serve to provide an additional base for the comment below. The tourmalines in Figures 1 and 2are worldwide examples of similar phenomena, wherein the early formed tourmaline appears tobe unstable with respect to the late formed rind and the interior portion has been hydrothermallyleached, altered, or replaced by other minerals.

For the past 9 years I have had the pleasure of being able to conduct basic pegmatite research atthe Little 3 Mine. During a ground verification phase to a GPR study, (Patterson, 1996), Iobserved the phenomena of schorl alteration to a sodium-iron rich, lithian bearing muscovite,(Stern, et al., 1986; Foord, et al., 1989; and Figures 3 to 10). The mine owner, Louis B.Spaulding, Jr., told me that Dr. Richard H. Jahns, a constant visitor to the mine between 1954and 1982, always said that they were muscovite pseudomorphs after tourmaline. Carefulobservation has proven this to be true. Apparently, late stage potassium rich hydrothermal fluidscirculated upward, from either the lower dyke or the aplite / line-rock unit, and altered theprimary schorl to muscovite, (Figure 3).

During the early stages of alteration, the muscovite A-axis followed the schorl C-axis, formingan apparent parallel intergrowth, (Figures 5, 6, 9, and 10). During the late stages of alteration, thetourmaline was almost completely removed, leaving the muscovite to crystallize in jack-strawfashion within the tourmaline shaped cavity left in the host albite, (Figures 4 to 8). Figures 9 and10 are photomicrographs of thin sections taken from a section of an altered tourmaline removedfrom the specimen shown in Figure 4. Apparently, the perthitic K-Feldspar in the footwall wasalso altered at this time, as it shows extensive leaching of albite along the rims of several crystalsin the specimen shown in Figure 4. The released elements from this alteration of tourmaline, (Na,

Fe, and B2O3), and associated Li, K, and H2O, appear to have been either incorporated into themuscovite or transported along the open pocket-line to form the new mineral boromuscovite, asexemplified by the material in the New Spaulding Pocket, (Stern, et. al., 1986; Foord, et. al,1989; and Foord, et. al., 1991). This alteration sequence is traceable for several meters updip andalong strike in the dyke. Following trends like these elsewhere, on the property, has led to thediscovery of other productive pockets, (L.B. Spaulding, Jr., personal communications, 1995-2002).

Alteration of tourmaline to muscovite or lepidolite is well known, as represented by thespecimens in Figures 11 to 14 and in London’s Figures 4 and 5. Perusal of the images in many ofthe issues of Mineralogical Record also depicts altered tourmalines, beryls, and other pegmatiteminerals. I will leave it to the geochemists to present the appropriate chemical explanation.

In answer to Mr. Rose’s question on why some parts of a pegmatite are altered while others arenot, I present an alteration sequence of a pegmatite in southern California that deals with theleaching of quartz from Graphic Granite intergrowths in the Garnet Ledge at the Little 3 Mine,(Figures 15 to 19). Here, in a 10 m dip-section of the dyke, all of the quartz was removed,leaving slightly altered perthitic K-feldspar and unaltered schorl, garnet, and apatite. The quartzand related hydrothermal solution appears to have been deposited updip as axinite and large, to30 cm, quartz crystals enclosing schorl needles, garnet, and axinite. Some of this fluid escapedthe confines of the pegmatite and reacted with the host granodiorite, (Patterson, 1997), to forman exocontact of very fine grained green muscovite and schorl-dravite, (Figure 18). In somepockets, cavernous beryl crystals were found, (Figure 19). Elsewhere in this same dyke, and inportions of the Topaz Ledge, similar sections containing quartz leached graphic granite can beobserved. Other features of the alteration sequences of the Garnet Ledge are beyond the scope ofthis comment.

If one considers the permeability of a crystallized pegmatite and the reactivity of therepresentative minerals, a common theme is observed. Where minerals are altered, they are opento exchange with late stage fluids. These fluids can either be trapped during progressivecrystallization or be introduced during later episodes of emplacement. The amount of alterationappears to be a function of the stability of that phase when in direct contact with thehydrothermal solution at that moment. If a mineral is buffered by an impermeable layer orencased in a non-reactive phase, then no alteration will occur. However, if the permeability ishigh enough for a significant amount of fluid to infiltrate a particular zone, then the possibilityexists to encounter a zone of altered material. In the case of the miarolitic pegmatites of SanDiego County, if a zone of alteration is encountered, then the optimum gem or specimen gradematerial is probably minimal, but most probably up-dip a zone of productive material maybefound. An early example of gemmy morganite beryl altering to bavenite was presented bySchaller and Fairchild, (1932). An apparently similar scenario has been the case with the recentfinds of pink and green tourmaline at the Cryo-Genie Mine, near Warner Springs,(http://home.earthlink.net/~goke/Cryo-Genie.htm).

I suggest to Mr. Rose that he complete a thorough 3-D geologic map of his pegmatite to look fortrends in mineralization and alteration. Some basic geochemistry might also be appropriate todelineate the levels of fractionation. I wish him luck with his endeavors and thank him for thisopportunity to express my observations.

References:

Cameron, E.N., Jahns, R.H., McNair, A.H. and Page, L.R., 1949, Internal Structure of GraniticPegmatites, Economic Geology, Monograph 2, 115 pp.

Foord, E.E., Martin, R.F., Fitzpatrick, J.J., Taggart, J.E., and Crock, J.G., 1991, Boromuscovite,a NEW member of the Mica Group, from the Little Three Mine Pegmatite, Ramona District, SanDiego County, California. American Mineralogist, 76, 1998-2002.

Foord, E.E., Spaulding, L.B., Jr., Mason, A.R., and Martin, R.F., 1989, Mineralogy andParagenesis of the Little Three Mine Pegmatites, Ramona District, San Diego CountyCalifornia. The Mineralogical Record, 20, 101-127.

Jahns, R.H. and Burnham, C.W., 1969, Experimental studies of pegmatite genesis: I. a model forthe derivation and crystallization of granitic pegmatites, Economic Geology, 64, 843-864.

Morgan, G.B.VI, and London, D., 1999, Crystallization of the Little Three layered pegmatite -aplite dike, Ramona District, California, Contributions to Mineralogy and Petrology, 136, 310-330.

Patterson, J.E., 1996, Modeling of Layered Aplitic Pegmatite Dikes Using Ground PenetratingRadar, Little Three Mine, Ramona District, San Diego County, California. The Spirit of Inquiry,University of Arizona Undergraduate Research Program, 17.

Patterson, J.E., 1997, The Little 3 Mine “Plug”; Evidence for a Zoned Plutonic Intrusion and aPlausible Parent Source of the Ramona Pegmatite District, Ramona, San Diego County,California, University of Arizona, unpublished B.Sc. Honors thesis, 152 pp.

Schaller, W.T., and Fairchild, J.G., 1932, Bavenite, a Beryllium Mineral, Pseudomorphous afterBeryl, from California, American Mineralogist, 17, 9, 409-422.

Sinkankas, J., 1988, Field Collecting Gemstones and Minerals, Geoscience Press, Tucson,Arizona, 397 pp.

Stern, L.A., Brown, G.E., Jr., Bird, D.K., Jahns, R.H., Foord, E.E., Shigley, J.E., and Spaulding,L.B., Jr., 1986, Mineralogy and Geochemical Evolution of the Little Three Pegmatite-aplitelayered intrusive, Ramona, CA, American Mineralogist, 71, 3 / 4, 406-427.

Thomas, R., Forster, H-J., and Heinrich, W., 2003, The behaviour of boron in a peraluminousgranite-pegmatite system and associated hydrothermal solutions: a melt and fluid-inclusionstudy, Contributions to Mineralogy and Petrology, 144, 457-472.

1)

Figure 1) Green elbaite from Brazil, frozen in quartz. The central portion of the crystal hasbeen altered and replaced by K-feldspar and white mica.

The tourmaline crystal is 3 cm dia x 6 cm in length.

2)

Figure 2) Tourmaline crystal from Malkhan, Russia. The central portion of the crystal hasbeen altered and replaced by K-feldspar. The exterior rind appears pristine.

The specimen is 4 x 7 cm.

3)

Figure 3) Cross Section of the Topaz Ledge, Little 3 Mine, Ramona, California. Themuscovite pseudomorphs after schorl are found in the lower intermediate unit in the MainDyke. See Stern, et. al., 1986, and Morgan and London, 1999, for chemical composition ofthe Main topaz bearing dyke. The pockets in the lower dyke appear to contain only schorl

and potassium feldspar with an infilling of zeolites.

4)

Figure 4) Exposure of the footwall portion of the “Pocket Line” of the Topaz Ledge, MainDyke, Little 3 Mine, Ramona, California. Geologic hammer for scale. (A = fine grainedalbite, K = altered potassium feldspar, slightly perthitic, M with white oval = muscovite

nest, Black Spots = schorl with or without muscovite intergrowths)

5A) Front 5B) Back

Figure 5 A & B) Specimen from the footwall of the “Pocket Line” from the region of themuscovite nest in Figure 2. Parallel muscovite – schorl alteration on the left side A & B.

Complete replacement with jackstraw muscovite flowers on the top of A. Spessartinegarnet is abundant in this region of alteration. Perthite on right side of A is altered with

plagioclase lamella leached out. Albite in graphic intergrowth with quartz, upper left of B.Specimen is 7 x 15 cm.

6)

Figure 6) Specimen from the footwall of the “Pocket-Line” from the region of the Ksection below the muscovite nest in Figure 2. The “Pocket-Line” is at the top of the

specimen Through-going tourmalines start as small solid flaring crystals at the bottom ofthe specimen, parallel muscovite-schorl alteration in the middle, and end as muscovite

flowers at the top.Specimen is 12 x 21 cm.

7) 8)

Figures 7 and 8) Details of muscovite flowers at top of altered schorl and extending into thefree space of the pocket line. Fragments of schorl remain in each specimen. The “stalks” of

the flowers maintain the tourmaline shape. Each specimen is 1 cm dia x 2 cm high.

9) 10)

Figures 9 and 10) Photomicrographs of a thin section of a schorl altered to muscovite fromthe specimen in Figure 6. The crystal is cut oblique to the C-axis. Mineral identification by

XRD at the University of Arizona.9) = PPL, 10) = XPL – Field of View = 2.5 mm

11) 12)

Figure 11) Muscovite pseudomorph after schorl. Unknown pegmatite in Maine. Specimenis 1 cm in dia x 1.8 cm high. Figure 12) Interior portion is a lepidolite pseudomorph after

copper bearing elbaite with copper bearing schorl outer rind, Paraiba, Brazil.Specimen is 1.5 x 5 x 7 cm.

13) 14)

Figures 13 and 14) Photomicrographs of a thin section of a copper bearing elbaite alteredto lepidolite from the specimen in Figure 12. The crystal is cut parallel to the C-axis.

Mineral identification by XRD at the University of Calgary.13) = XPL, 14) = PPL – Field of View = 2.5 mm

15)

Figure 15) Garnet Ledge, Little 3 Mine, Ramona, California, main cut looking Northwest.The dyke is approximately 2 m thick in this view. Quartz has been leached from the

Graphic Granite along the pocket-line.

16)

Figure 16) Graphic Granite (Perthite) from the Garnet Ledge, Little 3 Mine, with includedschorl. All of the quartz has been leached out.

The specimen is 6 x 9 – cm and was taken from the zone shown in Figure 15.

17)

Figure 17) Graphic Granite (Perthite) with albite, schorl, and garnet from the GarnetLedge, Little 3 Mine. All of the quartz has been leached out.

The specimen is 5 x 7 cm.

18)

Figure 18) Exocontact of very fine grained muscovite and schorl-dravite from the uppercontact zone of the Garnet Ledge, Little 3 Mine, Ramona, California, with the overlying

host granodiorite.

19)

Figure 19) Cavernous goshenite beryls from the Garnet Ledge, Little 3 Mine. Both crystalsshow extensive dissolution features on the interior and exterior. It is possible that both

beryl and intergrown quartz have been removed.Specimen on the left is two parallel growth crystals and is 3.2 x 3 cm.

Specimen on the right is coated with Bavenite, by XRD at the University of Arizona, and is 2.6 x 2.5 cm.